It seems to me that a lot of people are incorrectly assuming that the developers have stopped building mechs for the game and that the only mechs you'll ever see are the ones they've announced. To be equally as frank as the OP; that's just silly talk!
A slightly more likely scenario is that there are a bunch of mechs already finished that we haven't seen. I mean, wouldn't you say the mech announcements demonstrate that farily clearly? That aside, for BETA we'll probably only see the mechs they've announced. If for no other reason than the age old keep-them-wanting-more philosophy.
And as John pointed out, there will almost certainly be mechs showing up post launch. Unless all the self-entitled cry babies that seem to be the majority pull the cord and bail because they don't get what they want. I hope they're just a vocal minority.
Edited by Tyr Gunn, 26 April 2012 - 08:50 AM.